The present invention generally relates to an electrophotographic copying apparatus and more particularly, to a heat roller fixing device for use in an electrophotographic copying apparatus, in which a toner image transferred onto a transfer material such as copy paper is fused to be fixed on the copy paper.
Generally, in conventional heat roller fixing devices of the above-described type, it has been so arranged that a heating roller having a heating means therein, and a pressure roller in pressure contact with the heating roller are provided for passing therebetween copy paper bearing a toner image thereon so as to fix such toner image onto the copy paper, while the surface of the heating roller being maintained to a predetermined temperature so that proper fixing of the toner image can be achieved. In the known fixing devices, it has been further arranged so that a thick paper copying switch is separately provided for copying on a thick copy paper larger in thickness than an ordinary copy paper (plain paper), e.g. a postal card or the like and, by turning on the switch, a surface temperature of the heating roller is set slightly higher than that for the plain paper. The switch is required to be provided because the temperature set for the plain paper is insufficient for fusion fixing of the toner image on the thick paper which absorbs a larger quantity of heat than the plain paper.
It is to be noted here that the plain paper is distinguished from the thick paper by the weight, and more specifically, in the case of a process speed of 130 mm/sec., the plain paper ranges from 58 to 89 g/m.sup.2 at a set temperature of 153.degree. C. while the thick paper is in the range of between 90 and 157 g/m.sup.2 at a set temperature of 163.degree. C.
In the conventional fixing devices, there has been such an inconvenience that, since a temperature is required to be set for the thick paper separately from that for the plain paper as described above, an additional circuit for setting the temperature is required to be provided and thus, durability of the heating roller is extremely deteriorated. Namely, when the surface temperature of the heating roller is increased, temperature of a core metal thereof rises considerably and thus, problems arise as to deterioration of bonding strength of adhesives provided between the surface material of the heating roller and the core metal thereof, etc.
Furthermore, the known fixing devices have such a disadvantage that, since a temperature detecting element is required to be provided for accurately measuring a wide range of temperature, cost of the fixing devices is undesirably raised together with provision of the above-described additional temperature setting circuit.
Moreover, the conventional fixing devices are defective in that, since the heating roller is maintained at high temperatures for a predetermined time period after the thick paper copying switch has been turned off, undesirable offset phenomenon on the surface of the heating roller, i.e. so-called high temperature offset phenomenon takes place when copying on the plain paper is performed immediately after the thick paper copying switch has been turned off, so that the predetermined waiting time is undesirably required to be provided before the copying on the plain paper is performed, thus resulting in an inconvenience in the operation of the device.
Meanwhile, in the known fixing devices of a type in which an offset preventing liquid such as a silicone oil is applied to the surface of a heating roller as disclosed in U.S. Pat. No. 4,045,165, there has been such disadvantages that consumption of the silicone oil increases in proportion to volatilization thereof due to the high set temperature and further, temperature in the copying apparatus is also raised by the high set temperature, so that various portions such as a photosensitive member or photoreceptor, etc. are adversely affected.